Merino vs Cotton vs Polyester: Which Hiking Shirt?
Merino, cotton, polyester: which t-shirt to actually wear for summer hiking
The technical t-shirt aisle today offers three main fabric families: cotton (often organic, sometimes blended), polyester (and its polyamide and recycled variants), and merino. The marketing arguments all sound similar: breathable, lightweight, odour-resistant, quick-drying, comfortable. In practice, each fabric has real strengths and measurable weaknesses. This article runs all three through seven objective criteria, to help choose by actual use rather than by storytelling.
The three fabrics on the table: technical definitions
Cotton: natural fibre, low technicity
Cotton is a plant fibre from the fruit of the cotton plant. Its internal structure is hollow and strongly hydrophilic: the fibre absorbs up to 27 times its weight in water. Cotton fabric has a soft hand, typical weight of 140 to 180 gsm in a t-shirt, and moderate wear resistance. It is mostly used in urban wear.
Polyester: synthetic fibre, chemical technicity
Polyester is a synthetic fibre derived from petroleum (or from recycled PET bottles). Its structure is smooth, tubular, hydrophobic. Modern technical versions are often structured to create a capillary effect that wicks sweat outward. Typical weight in technical t-shirts, 100 to 150 gsm. High abrasion durability.
Merino: ultra-fine animal fibre, biological technicity
Merino is a wool from a sheep breed selected for fleece fineness. Fibre diameter between 16.5 and 19.5 microns for technical textile use. The fibre is made of keratin, structurally wavy (crimp), and hygroscopic (absorbs water vapour inside the fibre). Typical weight in summer t-shirt, 140 to 170 gsm. Medium durability, specific care.
Criterion 1: sweat management under effort
Cotton absorbs moisture at the surface, holds it, saturates quickly and becomes heavy. Immediate wet-fabric feel, very slow evaporation, sudden cooling at rest. For sustained effort, cotton offers no satisfactory technical solution and becomes problematic on exposed ridges.
Polyester wicks sweat outward through capillary action. Efficient management of surface moisture, but the fabric stays wet on the outer surface as long as the environment does not allow evaporation. "Wet but not heavy" feel.
Merino absorbs water vapour inside the fibre (up to 35% of its weight), holds it, then releases it through progressive evaporation. Prolonged dry-skin feel, fabric that does not become heavy abruptly. Performance above cotton, different from polyester: slower at surface wicking, but without the wet-surface feel.
Criterion 1 verdict: polyester and merino tied on short effort. Merino ahead on long effort thanks to absence of wet feel. Cotton far behind.
Criterion 2: drying speed after washing or rain
Cotton dries very slowly: 8 to 12 hours in open air, longer in humid conditions. Impractical for an evening wash on a trek if the next day's stage starts early.
Polyester dries very quickly: 1 to 3 hours in open air. Best performer on this criterion.
Merino dries moderately: 4 to 6 hours in open air in normal conditions, longer in high humidity. Sufficient for an evening wash and morning use.
Criterion 2 verdict: polyester clearly ahead. Merino acceptable. Cotton disqualified.
Criterion 3: body odour management
Sweat itself has no smell. Odour appears when bacteria metabolise the organic compounds of sweat. Not all textiles offer them the same hospitality.
Cotton is relatively odour-neutral for short use, but its persistent moisture creates a favourable environment for bacterial growth over several days.
Polyester is the most odour-prone fabric. Its smooth and oleophilic structure welcomes particularly well the bacteria responsible for isovaleric acid. Marked smell from 24 hours under effort.
Merino is the most odour-resistant fabric. The keratin surface is biochemically hostile to bacteria, and internal moisture absorption limits growth. Multi-day wear without marked odour, decisive difference in hut dormitories.
Criterion 3 verdict: merino clearly ahead. Cotton acceptable on short use. Polyester behind.
Criterion 4: direct skin comfort
Cotton is soft to the touch, skin-friendly, suitable for sensitive skin. No prickle, no irritation. Historical reference for the comfortable t-shirt.
Modern polyester is decent on skin but keeps a slightly plastic or slippery feel. Some finishes create micro-irritations under sustained effort, particularly on friction zones (backpack, sports bra).
Extra-fine merino (micron below 19) is extremely soft, comparable to cotton, with a slightly drier and more "alive" hand. Standard merino (22 microns and above) can prickle on sensitive skin. The 140 gsm at 17.5 microns used on Bjork MC 140 and Finn MC 140 falls in the non-prickling category.
Criterion 4 verdict: cotton and extra-fine merino tied. Polyester slightly behind.
Criterion 5: durability and longevity
Cotton holds up decently to wear but can deform and fade quickly depending on weave quality. Its real lifespan under intensive use rarely exceeds 2 to 3 seasons.
Polyester is the most abrasion-resistant. It keeps its shape, colours and structure for many years. Downside: it releases plastic microfibres with every wash, a documented environmental problem.
Merino is more vulnerable than polyester to snags (thorns, poorly fitted backpack, rings), but preserves its shape, thermal regulation and anti-odour properties over many seasons if cared for properly. Quality 140 gsm t-shirts have a real lifespan of 5 to 8 years in regular use.
Criterion 5 verdict: polyester ahead on raw mechanical resistance. Merino ahead on preservation of technical properties. Cotton behind.
Criterion 6: environmental impact
Conventional cotton is one of the most water- and pesticide-intensive crops in the world. Organic cotton significantly reduces these impacts but remains very water-hungry. Biodegradable at end of life.
Virgin polyester comes from petroleum, emits CO2 in production, releases microplastics at every wash, and is not biodegradable. Recycled polyester reduces production footprint but keeps the other two problems.
Merino is a renewable, biodegradable fibre. Sheep farming has a real carbon footprint (enteric methane, land use) but limited if practices are extensive. The mulesing-free label (which Fjork guarantees via AWTA certification, Australian Wool Testing Authority) excludes farming practices problematic for animal welfare. Fjork Merino is an independent brand based in Sion, Switzerland.
Criterion 6 verdict: mulesing-free merino and organic cotton roughly equivalent on the selected indicators. Polyester behind on the long term (microplastics, end of life).
Criterion 7: price relative to real use
Cotton is the cheapest to buy: 20 to 50 euros for a decent t-shirt. Over a lifespan of 2 to 3 seasons in technical use, annual cost of 7 to 20 euros.
Technical polyester is at 40 to 100 euros depending on brand. Lifespan of 3 to 5 seasons, annual cost of 8 to 25 euros.
Technical merino is the most expensive to buy: 70 to 130 euros for a quality 140 gsm t-shirt. Lifespan of 5 to 8 seasons, annual cost of 10 to 25 euros.
Criterion 7 verdict: over intensive real use and over time, the three fabrics end up in a very similar annual cost range. Cotton loses its price advantage over time, merino loses its price disadvantage over time.
Synthesis and verdict by use profile
Visual synthesis of the 7 criteria:
- Sweat long effort: merino > polyester > cotton
- Drying: polyester > merino > cotton
- Anti-odour: merino > cotton > polyester
- Skin comfort: cotton = extra-fine merino > polyester
- Technical durability: polyester = merino > cotton
- Environmental impact: mulesing-free merino ≈ organic cotton > polyester
- Real annual cost: fabrics equivalent over time
Profile urban use and short outings: organic cotton remains a legitimate choice. Comfort, low price, no odour issue over a day. Technical polyester and merino are oversized for this use.
Profile day hiking and short sport sessions (under 3h): technical polyester is a good price-performance compromise. Merino brings superior comfort but is less economically justified on this use.
Profile alpine day hiking in heat (over 3h, over 25°C): merino takes the advantage through sweat management without wet feel. Cotton disqualified.
Profile multi-day trek, thru-hiking, self-supported travel: merino is the most coherent fabric with the use format. Anti-odour and strategic pack lightening largely compensate purchase price and drying time. Cotton disqualified, polyester acceptable but dominated.
Profile short intense dry sport (track running, air-conditioned gym): technical polyester remains relevant. Merino brings different comfort, but not necessarily superior on this format.
The choice is therefore not ideological. It is a matter of fabric-use match. For most summer outdoor practices in self-supported format, 140 gsm merino is the most coherent technical answer. For urban use, organic cotton remains valid. For short intense sport, polyester keeps its place.
